The fungal vacuole is an acidic organelle that contains many hydrolases, including several proteases, and is essentially equivalent to the mammalian lysosome. Several of the hydrolases have been identified and characterized in one or more species of fungi, particularly in yeast; these include protease A(PEP4 or PrA), protease B(PrB), aminopeptidase(APE), dipeptidyl aminopeptidase B(DPAP B), carboxypeptidase Y(CPY), and carboxypeptidase S(CPS). Most of the vacuolar hydrolases are glycoproteins which are synthesized as inactive precursors. In fact, all the aforementioned proteases with the exception of APE have signal peptides that lead to transit through the secretory pathway. In the late golgi, vacuolar proteins are sorted from secretory proteins and eventually delivered to the vacuole. In addition to a signal peptide, most vacuolar proteins also have a propeptide which is cleaved upon delivery to the vacuole to generate the mature active enzyme. It has been demonstrated that the amino acid information in PrA and CPY required for vacuolar targeting is present within the propeptide(Johnson et al., Cell 48: 875-885, 1987; Rothman et al. PNAS USA 83: 3248-3252, 1989; Valls et al., Cell 48: 887-897, 1989; Valls et al. J. Cell Biol. 111: 361-368, 1987). For CPY a string of four amino acid residues (QRPL) has been shown to be required for localization to the vacuole (Valls et al., J. Cell Biol. 111: 361-368, 1990). Once delivered to the vacuole, proteinase A (pep4)cleaves the propeptide of CPY and PrB leading to the activation of the proteases (Ammerer et al., Mol. Cell. Biol. 6: 2490-2499, 1986; Woolford et al., Mol. Cell. Biol. 6: 2500-2510, 1986).
In S. cerevisiae, three classes of mutants which mislocalize or missort vacuolar proteins have been identified (Bankaitis et al., PNAS USA 83: 9075-9079, 1986; Robinson et al., Mol. Cell. Biol., 8: 4936-4948, 1988; Rothman et al.,EMBO J. 8: 2057-2065, 1989; Rothman and Stevens, Cell 47: 1041-1051, 1986). These mutants are called vps or vacuolar protein sorting mutants. Several of these mutants are isolated using a selection based on the observation that overexpression of a vacuolar protease due to a high copy number on a plasmid leads to a secretion of vacuolar proteases (Stevens et al., J. Cell Biol. 102: 1551-1557, 1986; Rothman et al, PNAS USA 83: 3248-3242, 1986). This suggests that it is possible to saturate the sorting machinery within the late golgi.
In S. cerevisiae, it has also been demonstrated that strains deleted for PEP4, CPY and PrB produce higher levels of heterologous proteins due to a decrease in proteolysis of the desired product. Therefore, the vacuolar proteases in question are important from a commercial point of view because many of the fungi which produce them are used for recombinant production of heterologous proteins. The presence of these proteases in fermentation is undesirable, in that they can degrade the protein of interest, thereby significantly reducing yield. Elimination of the function of any given protease is facilitated by the disruption or deletion of the gene encoding it; however, doing so first requires identification and isolation of the corresponding gene in the host species of interest. As noted above, a few such genes have been isolated from various yeast strains; however, the genes encoding vacuolar proteases in the filamentous ascomycetes or deuteromycetes are less well known. For example, PEPC(Frederick et al., Gene 125: 57-64, 1993) and PEPE (Jarai et al., Gene 145: 171-178, 1994) genes coding for two other vacuolar proteases from Aspergilus niger have been isolated. PEPC codes for a proteinase B(PrB) homologue, and PEPE codes for a proteinase A homologue. The gene PEP4 from Neurospora crassa coding for a PrA homologue has also been cloned(Bowman, 17th Fungal Genetics Conference, 1993). For the first time herein is described the gene encoding a vacuolar CPY from a filamentous ascomycete or deuteromycete.